Stanford University PhD Dissertation Defense
PhD Candidate : Szu-Lin Cheng
Title : Germanium as an Infrared Optical Emitter for Monolithic Integration
on Silicon
Research Advisor : Prof. Yoshio Nishi
Date : Wednesday, Oct 20th, 2010
Time : 10 am (Refreshments served at 9:40 am)
Location : CISX Auditorium (101X)
http://cis.stanford.edu/misc/directions.html
A silicon (Si) compatible laser for applications in telecommunication and
optical interconnect systems has been an interesting topic for several years
now, but has yet to be practically demonstrated. The main problem is finding
an appropriate lasing material at 1550 nm which can be monolithically
integrated on silicon with conventional CMOS processes. Germanium (Ge) is
compatible with Si and has a direct band gap of 0.8 eV, corresponding to the
required optical communication wavelength of 1550 nm. The small difference
of 0.134 eV between the direct and indirect band gaps of Ge suggests the
possibility of a radiative direct band gap transition. Strategies to improve
the luminescence properties of germanium have included large tensile strain,
tin alloying, and electron band filling. In this talk, we focus on the last
approach since the emission wavelength from such a method stays near the
desired 1550 nm.
We first show how Ge direct band emission can be improved by using electron
band filling of the conduction band. To achieve high electron band filling,
an in-situ doping technique was applied during the growth of epitaxial Ge on
Si. A strong enhancement from direct band photoluminescence (PL) was
observed from highly-doped (1E19 cm-3) n-type epi-Ge, demonstrating that
electron band filling improves the direct band emission strength. We then
successfully demonstrate room temperature direct band electroluminescence
(EL) from Ge n+/p light emitting diodes (LED) on a Si substrate, which is a
key step towards a CMOS-compatible laser. The contribution of electron band
filling and the temperature dependence to the device efficiency will also be
discussed. Additionally, we fabricate and optically characterize epitaxial
Ge microdisks on Si. These mircodisk resonators are successfully coupled to
fiber tapers and display clear whispering gallery modes (WGM) in
transmission as well as photoluminescence. Finally, we combined the LED
structure and the microdisk cavity to demonstrate an electrically-pumped Ge
resonator diode. Both our optical and electrical resonators are currently
limited by the Ge doping concentration, which prevents sufficient electron
band filling to allow material gain or lasing. Possible solutions to this
problem will also be discussed.
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